Network Working Group Y. YONEYA, Ed. Internet-Draft K. Fujiwara, Ed. Expires: September 7, 2006 JPRS March 6, 2006 Downgrading mechanism for Internationalized eMail Address (IMA) draft-yoneya-ima-downgrade-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 7, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract Traditional mail system handles only US-ASCII characters in SMTP envelope and mail headers. The Internationalized eMail Address (IMA) is implemented by allowing UTF-8 characters in SMTP envelope and mail headers. To deliver IMA through IMA incompliant environment, some sort of converting mechanism (i.e. downgrading) is required. This document describes requirements for downgrading, SMTP session downgrade, header downgrade and implementation consideration. YONEYA & Fujiwara Expires September 7, 2006 [Page 1] Internet-Draft IMA Downgrade March 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Downgrade Requirements . . . . . . . . . . . . . . . . . . . . 4 3.1. Timing and conditions of downgrading . . . . . . . . . . . 4 3.2. Mail Header Downgrading . . . . . . . . . . . . . . . . . 4 3.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 4 4. SMTP Downgrading . . . . . . . . . . . . . . . . . . . . . . . 5 5. SMTP DATA/Header downgrading . . . . . . . . . . . . . . . . . 6 5.1. Downgrading with MIME encapsulation . . . . . . . . . . . 6 5.2. Header conversion . . . . . . . . . . . . . . . . . . . . 8 6. Implementation consideration . . . . . . . . . . . . . . . . . 8 6.1. MUA . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.2. MDA Requirements . . . . . . . . . . . . . . . . . . . . . 9 7. Security considerations . . . . . . . . . . . . . . . . . . . 9 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 10. Normative References . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Intellectual Property and Copyright Statements . . . . . . . . . . 13 YONEYA & Fujiwara Expires September 7, 2006 [Page 2] Internet-Draft IMA Downgrade March 2006 1. Introduction Traditional mail system which is defined by [RFC2821] and [RFC2822] allows US-ASCII characters in SMTP envelop and mail headers. IMA proposal [IMA-Framework],[IMA-UTF8], [IMA-SMTPext] allows UTF-8 characters in SMTP envelop and mail headers. Carrying IMA from sender to recipients requires all components on the mail delivery route are IMA compliant. Otherwise IMA can't be delivered. To solve the problem, this document describes downgrade mechanism that enables delivering IMA by converting it to corresponding US-ASCII representation on current mail delivery system. Not only SMTP envelope, but also UTF-8 in mail headers MUST be converted to US-ASCII. Converting IMA to US-ASCII SHOULD base on algorithmic method which is proposed by [IMA-SMTPext]. Downgrading in IMA consists from following two parts: o SMTP session downgrade o header downgrade In this document, requirements for downgrading is described in section Section 3, SMTP session downgrade is described in Section 4, and header downgrade is described in Section 5. 2. Terminology This document assumes a reasonable understanding of the protocols and terminology of the core email standards as documented in [RFC2821] and [RFC2822]. Much of the description in this document depends on the abstractions of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"). However, it is important to understand that those terms and the underlying concepts postdate the design of the Internet's email architecture and the "protocols on the wire" principle. That email architecture, as it has evolved, and the "wire" principle have prevented any strong and standardized distinctions about how MTAs and MUAs interact on a given origin or destination host (or even whether they are separate). The final ("delivery") MTA stores Mail messages in a "message store" or resends messages where the receiver has assigned. In this document, this function is called Mail Delivery Agent(MDA). In this document, an address is "all-ASCII" if every character in the YONEYA & Fujiwara Expires September 7, 2006 [Page 3] Internet-Draft IMA Downgrade March 2006 address is in the ASCII character repertoire [ASCII]; an address is "non-ASCII" if any character is not in the ASCII character repertoire. The term "all-ASCII" is also applied to other protocol elements when the distinction is important, with "non-ASCII" or "internationalized" as its opposite. The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 3. Downgrade Requirements 3.1. Timing and conditions of downgrading This section describes timing and conditions of downgrading. o Timing: SMTP client detects SMTP server doesn't support "IEmail" option at EHLO. [IMA-SMTPext] o Conditions: SMTP client detects UTF-8 is included in SMTP envelope or mail headers. Note that SMTP client SHOULD check headers in outgoing SMTP DATA whether they include 8bit (UTF-8) data. 3.2. Mail Header Downgrading Target of downgrading elements in mail headers (SMTP data) are below: Originator address(es): IMA in From, Reply-To and Sender and their Resent- headers MUST be target of downgrading. Destination address(es): IMA in To and Cc and their Recent- headers MUST be target of downgrading. IDs: IDs such as Message-ID, In-Reply-To and Referenece MUST NOT be target of downgrading. other headers: UTF-8 in other headers such as Subject and Received SHOULD be target of downgrading. 3.3. Requirements 1. Downgrading MUST be performed only once. 2. Upgrading MUST be performed at minimized place such as final destination like receipient MUA. 3. Downgrading and upgrading MUST be automated. 4. Downgrading and upgrading MUST be easy and lightweight as it is possible to do with MTA like 8BITMIME encapsulation. 5. Downgrade and upgrade method MUST be defined clearly. YONEYA & Fujiwara Expires September 7, 2006 [Page 4] Internet-Draft IMA Downgrade March 2006 6. Downgrading and upgrading MUST preserve all header information. 7. Downgrading MUST support sender authentication header checking such as SPF and DomainKeys/DKIM. 8. Downgrading occurrence MUST be recorded. 4. SMTP Downgrading Downgrading MUST be performed in each SMTP session. Target of downgrading elements in SMTP envelope are below: o MAIL FROM: o RCPT TO: Downgrading in SMTP envelope uses ALT-ADDR and ATOMIC option proposed in [IMA-SMTPext]. MUA of mail sender MUST append ALT-ADDR or ATOMIC option to all envelope from (MAIL FROM) and envelope to (RCPT TO) to denote alternative US-ASCII address when sending mail. When MUA/MTA is transfering mail and finds its envelope is IMA, it MUST decide to bounce or downgrade if receiving MTA is IMA incompliant. Further, even if no downgrading is performed for envelope from/to, MUA/MTA SHOULD downgrade headers including UTF-8. This is described in next session. MTA MAY downgrade messages that envelope from/to of IMA have ALT-ADDR with alternative US-ASCII address or ATOMIC is "y". MTA generates alternative US-ASCII address when ALT-ADDR option is not specified and ATOMIC is "y". Alternative US-ASCII address generation algorithms are below: domain-part: Punycode/IDNA local-part: Punycode without normalization. Prefix MUST be assigned by IANA (which is not "xn--"). MTA replaces IMA with specified or generated alternative US-ASCII address. Then appends replaced information with IMA-Downgraded-From and IMA-Downgraded-To header in mail header (outgoing SMTP DATA). IMA-Downgraded-From: IMA-Downgraded-To: Note that when downgrading, not to disclose whole recipient address, MUA/MTA SHOULD make SMTP connection per each receipient address. YONEYA & Fujiwara Expires September 7, 2006 [Page 5] Internet-Draft IMA Downgrade March 2006 Also note that by appending IMA-Downgraded-From/To headers, MUA/MTA MUST perform SMTP/Header downgrading. This is described in next section. Downgraded envelope to is parsed only in MDA and delived to final mailbox. Case study: SPF check SPF checks envelope from's domainname and smtp connection IP address. If ALT-ADDR's domainname is Punycode/IDNA of IMA domainname, it is equal to SPF/IMA (need to define). In this case, SPF check will be performed correctly. Otherwise, more detailed consideration is required. 5. SMTP DATA/Header downgrading In this section, two methods for SMTP DATA/Header downgrading is proposed. One is to encapsulate whole data, and other is to translate header by header. 5.1. Downgrading with MIME encapsulation This downgrade method requires new MIME 'Content-Type:' which express EAI(Email Address Internationalization). This document assumes 'Content-Type: Message/EAI' existence. Downgrade/Encoding * If mail header contains UTF-8 data, downgrade whole message to be MIME encoded. Whole message becomes new MIME part (Message/ EAI). * Originator Addresses (From, Sender, etc.), Destination Addresses (To, CC, etc.), IDs (Message-ID, etc.), Subject, Date headers are copied from original header. * If From header contains IMA, it is replaced with downgraded Envelope-from. * If To or CC headers contain IMA, they are replaced with single downgraded envelope-to as To header. * If Subject header contains UTF-8, it is rewrited to a certain message or encoded by RFC2047. * Message-ID, Date headers are preserved. As a result, new body contains one MIME part (Message/EAI). YONEYA & Fujiwara Expires September 7, 2006 [Page 6] Internet-Draft IMA Downgrade March 2006 Encoding example Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: Multipart/Mixed; boundary="--Next_Part(unique_string)--" Content-Transfer-Encoding: 8bit Subject: DOWNGRADED_SUBJECT From: DOWNGRADED_FROM To: DOWNGRADED_TO Date: DATE ----Next_Part(unique_string)-- Content-Type: Message/EAI Content-Transfer-Encoding: 8bit Content-Disposition: inline ENTIRE_ORIGINAL_MAIL IMA-Downgraded-From: IMA-Downgraded-To: Received: ... Received: ... Message-Id: MESSAGE_ID Mime-Version: 1.0 Subject: UTF-8_SUBJECT From: IMA To: IMA Date: DATE MAIL_BODY ----Next_Part(unique_string)---- Figure 1 Upgrade/Decode * If mail message contains only one MIME part and its Content- Type is 'Message/EAI', it may be downgraded. To check if downgraded, compare mail body's message-id and MIME part's message-id. If message-ids are same, it is downgraded message. Then, treat MIME part as entire mail message. * When checking trace field, checker SHOULD check Received header both in wrapping headers and headers in encapsulated part. Case study: DomainKeys/DKIM DomainKeys/DKIM checker performs upgrade/decode downgraded message first. YONEYA & Fujiwara Expires September 7, 2006 [Page 7] Internet-Draft IMA Downgrade March 2006 Pros: * MTA does not need to decode each headers carefully. * Whole headers can be submitted AS IS. Cons: * IMA from/to can not distinguish from encoded mail headers. * IMA incompliant MUA can not treat encoded message. 5.2. Header conversion Define conversion method to US-ASCII for all headers which contains IMA. Each header has its own downgrading method. Basically, MIME encoding of RFC 2047. Receipient/Sender addresses and Received headers which may contain IMA need special processing. Downgrading mail addresses in mail header: Extract every addr-spec [RFC2822] of mailboxes from mail headers including UTF-8. For each addr-spec, if it includes UTF-8, convert it into ACE with the same method described in Section 4. Original IMA SHOULD remain as a comment encoded by base64 of MIME with UTF-8 tag. Note that some special characters in addr-spec MUST be escaped. If mailbox elements except for addr-spec include UTF-8, those MUST be encoded by base64 with UTF-8 tag. Downgrading other data: Encode UTF-8 part of headers by base64 of MIME with UTF-8 tag. Pros: * IMA incompliant MUA can display mail body except for original IMA from/to. Cons: * Implementation is difficult because MUA/MTA must parse each header and encode it by defined method. * Hard to preserve whole information AS IS. Therefore, to check DomainKeys/DKIM requires special consideration. 6. Implementation consideration 6.1. MUA IMA compliant MUA MUST implement downgrade mechanism for sending. MUA MAY encode UTF-8 in Subject header with the same encoding of body part while downgrading. IMA compliant MUA MUST decode downgraded mail and MUST show IMA on display. YONEYA & Fujiwara Expires September 7, 2006 [Page 8] Internet-Draft IMA Downgrade March 2006 6.2. MDA Requirements This section describes downgrading in MDA. 1. MDA MUST NOT convert downgraded header to UTF-8. 2. Record Return-Path header in ACE form. 3. Perform downgrading for each Storage/Back-end-Process. If and only if MDA knows MUA is IMA compliant, then no downgrading is performed. 4. If MDA detects that SMTP recipient address is downgraded IMA, then MDA MUST decode IMA and perform the same processing as if it were IMA. MDA MAY normalize or canonicalize local-part before processing it. 7. Security considerations See the extended security considerations discussion in [IMA- Framework] 8. IANA Considerations To distinguish downgraded IMA in ACE form, it MUST have ACE-Prefix. The ACE-Prefix MUST be differ from IDNA ACE-Prefix to avoid possible confusion. IANA will assign IMA ACE-Prefix when RFC is published. 9. Acknowledgements John Klensin, Harald Alvestrand, Yangwoo Ko, YAO Jiankang, Jeff Yeh, James Seng and JET members. 10. Normative References [ASCII] American National Standards Institute (formerly United States of America Standards Institute), "USA Code for Information Interchange", ANSI X3.4-1968, 1968. ANSI X3.4-1968 has been replaced by newer versions with slight modifications, but the 1968 version remains definitive for the Internet. [Hoffman-IMAA] Hoffman, P. and A. Costello, "Internationalizing Mail Addresses in Applications (IMAA)", draft-hoffman-imaa-03 (work in progress), October 2003. [IMA-Constraints] YONEYA & Fujiwara Expires September 7, 2006 [Page 9] Internet-Draft IMA Downgrade March 2006 Klensin, J., "Internationalization in Internet Applications: Issues, Tradeoffs, and Email Addresses", draft-klensin-ima-constraints-00 (work in progress), Febrary 2006. [IMA-Framework] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", draft-klensin-ima-framework-01 (work in progress), February 2006. [IMA-SMTPext] Yao, J., Ed., "SMTP extension for internationalized email address", draft-yao-smtpext-02 (work in progress), Febrary 2006. [IMA-UTF8] Yeh, J., "Internationalized Email Headers", draft-yeh-ima-utf8headers-01 (work in progress), February 2006. [JET-IMA] Yao, J. and J. Yeh, "Internationalized eMail Address (IMA)", draft-lee-jet-ima-00 (work in progress), June 2005. [Klensin-emailaddr] Klensin, J., "Internationalization of Email Addresses", draft-klensin-emailaddr-i18n-03 (work in progress), July 2005. [RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [RFC1651] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extensions", RFC 1651, July 1994. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. YONEYA & Fujiwara Expires September 7, 2006 [Page 10] Internet-Draft IMA Downgrade March 2006 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003. [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. YONEYA & Fujiwara Expires September 7, 2006 [Page 11] Internet-Draft IMA Downgrade March 2006 Authors' Addresses Yoshiro YONEYA (editor) JPRS Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda Chiyoda-ku, Tokyo 101-0065 Japan Phone: +81 3 5215 8451 Email: yone@jprs.co.jp Kazunori Fujiwara (editor) JPRS Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda Chiyoda-ku, Tokyo 101-0065 Japan Phone: +81 3 5215 8451 Email: fujiwara@jprs.co.jp YONEYA & Fujiwara Expires September 7, 2006 [Page 12] Internet-Draft IMA Downgrade March 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. YONEYA & Fujiwara Expires September 7, 2006 [Page 13]